feat(tests): add --preprocess-dir CLI option to pytest

- conftest.py: registra --preprocess-dir via pytest_addoption
- test_preprocessors.py: usa pytest_generate_tests per leggere i casi
  a collection time con accesso a config; _content e _fixtures_dir
  accettano path dinamico

Usage: pytest tests/test_preprocessors.py --preprocess-dir /my/folder

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/conftest.py

@@ -4,6 +4,14 @@ Provides an async SQLite in-memory engine that auto-creates all tables,
 a per-test session, and a FastAPI ``TestClient`` wired to use it.
 """
 
+
+def pytest_addoption(parser):
+    parser.addoption(
+        "--preprocess-dir",
+        default=None,
+        help="Override fixture folder for preprocessor tests (must contain cases.yaml + data/)",
+    )
+
 from __future__ import annotations
 
 import json